1. Technical Field
The disclosed embodiments are directed generally to internal combustion engines, and, in particular, to diesel engines utilizing an additional fuel for reduced emissions.
2. Description of the Related Art
A spark-ignition engine introduces the fuel with the intake charge air to achieve good combustion at all loads and speeds. These engines can operate with gasoline, alcohol, or other low-cetane, high-octane fuels. However, they are limited to a relatively low compression ratio, e.g. 9 to 1, in order to limit the potential for compression of the mixture to cause pre-ignition of the fuel before the spark fires.
Diesel engines introduce the fuel after the charge air has been compressed, which prevents the possibility of pre-ignition and thereby allows the engine to operate at a higher compression ratio (e.g. 16 to 1 or higher), where greater fuel conversion efficiency can be achieved. Diesel engines utilize high-cetane, low-octane diesel fuels which easily ignite when directly injected into the compressed charge air. However, direct injected fuel often combusts insufficiently to fully oxidize the fuel, causing harmful particulate matter (carbon particles, also known as smoke or soot) to be present in the exhaust. Also, because the higher compression ratio and combustion temperature can encourage nitrogen-oxygen bonding, diesel engines can also produce high levels of harmful nitrogen oxide emissions (NOx).
It has been a long-standing objective in the art to minimize particulate matter and NOx emissions from diesel engines. In particular, for diesel engines it is important to achieve high efficiency high load and speed engine operating conditions with low exhaust particulate/smoke content while maintaining low NOx formation.
Particulate formation occurs when the large, carbon-intensive molecules of conventional diesel fuel are unable to fully oxidize due to insufficient local availability of oxygen, or have insufficient time to access the oxygen during the combustion process. This most often occurs at medium and higher loads (where more diesel fuel molecules and their multiple carbon-to-carbon bonds must be oxidized), for example, beginning about 4 bar BMEP (brake mean effective pressure) and beginning to reach unacceptable levels between about 6 to 8 bar BMEP, and at higher engine speeds (where there is less time for diesel fuel molecules to access oxygen). Fuels comprising molecules with lower carbon content would require less local oxygen to completely oxidize. Also, fuels that also include oxygen (e.g., alcohol fuels such as methanol, CH3OH) would require even less local oxygen to result in good oxidation.
Various methods of charge air dilution can help achieve lower NOx production. For example, it is known that limiting the intake oxygen concentration levels, e.g. to 11-13%, is required for a diesel engine to hit the EPA NOx target of 0.2 grams per brake horsepower-hour engine-out at higher loads. Certain patents issued to applicant relate to this goal, including U.S. Pat. No. 6,857,263 (Gray Jr. et al., “Low Emission Diesel Combustion System With Low Charge-Air Oxygen Concentration Levels and High Fuel Injection Pressures”).
One indicator that complete combustion has been accomplished is the degree to which the carbon content of the fuel molecule has been converted to an oxidized form, specifically carbon monoxide (CO), which is partially oxidized, or carbon dioxide (CO2), which is completely oxidized. Ideally, all the carbon content of the fuel would be completely oxidized as CO2. Further, since CO is a gas under engine exhaust conditions, small quantities of CO in the exhaust are more easily completely oxidized than elemental carbon (i.e. particulate/smoke) or the carbon in high carbon content fuel molecules.
Adding a second, complement fuel to the intake charge could make oxidation more complete. The term “complement” fuel is used herein to indicate a second fuel that is used together with the diesel fuel in combustion, and which is generally not the preferred fuel in the event of single-fuel combustion. The method of diesel engine fumigation, in which a liquid or gaseous fuel is added as a complement fuel to the charge air, is known in the art. Prior art dual fuel engines, for example those using diesel fuel with a complement fuel, generally add relatively low levels of the second fuel (e.g. 30% or less by volume) to diesel, with little or no intake oxygen reduction (i.e. charge dilution). If larger quantities of the second fuel were used, it would result in early combustion initiation and severe engine knock due to the higher compression ratio present in a diesel engine. The invention herein prevents this problem by diluting the charge air so as to reduce the concentration of oxygen to below that which would support initiation of combustion of the second fuel on compression.